at reducing the sulfur content of Al-Ahdab crude oil prior to processing by ... The effect of operation variables on the oxidation desulfurization of crude oil.
R REPUPLIC OF IRAQ I MINISTRY OF HIGHER EDUCATION AND SCIENTIFIC RESEARCH UUNIVERSITY OF TECHNOLOGY B BAGHDAD - IRAQ
Desulfurization of Al-Ahdab Crude Oil using Adsorption-Assisted Oxidative Process
A Thesis Submitted to the Chemical Engineering Department of the University of Technology in a Partial Fulfillment of the Requirements for the Degree of Master of Science in Chemical Engineering
BY
Saja Mohsen Jabbar B.Sc. in Chemical Engineering July, 2013
صدق اهلل العلي العظيم (سورة هود :اآلية )88
SUPERVISOR CERTIFICATION
I certify that this thesis entitled (Desulfurization of Al-Ahdab
Crude Oil using Adsorption-Assisted Oxidative Process) presented by Saja Mohsen Jabbar was prepared under my supervision in a partial fulfillment of the requirements for the degree of Master of Science in Chemical Engineering at the Chemical Engineering Department, University of Technology.
Signature: Prof. Dr. Neran Khalel Ibrahim Data:
/ / 2013
In view of the available recommendations I forward this thesis for debate by the Examining Committee.
Signature: Asst. Prof. Dr. Qusay F. Abdal Hameed (Deputy Head of Department For Scientific and Postgraduate Affairs) Data: /
/ 2013
Linguistic Supervisor Certification This is to certify that I have read the thesis entitled ( Desulfurization
of Al-Ahdab Crude Oil using Adsorption-Assisted Oxidative Process) and corrected any grammatical mistakes I found. The thesis is, therefore, qualified for debate.
Signature:
Name: Asst. Prof. Eyad Shamseldeen Date:
/ / 2013
Certificate We certify, as an examining committee, that we have read this thesis entitled (Desulfurization of Al-Ahdab Crude Oil using Adsorption-Assisted Oxidative Process) examined the student (Saja Mohsen Jabbar) in its content and found that the thesis meets the standard for the degree of Master of Science in Chemical Engineering. Signature :
Signature:
Prof. Dr. Neran K. Ibrahim
Asst. Prof. Dr. Nada S. Ahmed Zeki
(Supervisor) Date: / /2013
(Member) Date: / /2013
Signature:
Signature:
Dr. Adel S. Hamadi
Asst. Prof. Dr. Adnan A. Jabbar
(Member)
(Chairman)
Date: / /2013
Date: / /2013
Approved by the Head of the Chemical Engineering Department Signature: Prof. Dr. Thamer J. Mohammed (Head of the Chemical Engineering of Department) Data: /
/ 2013
Dedication To My Kind Father and Mother Brothers and Sisters & Every one helped me during study I present my deep respect and love I could not have done all this without you.
Saja
Acknowledgments
Acknowledgments
First of all I thank Allah who gives me patience and strength to continue……. Secondly, I would like to express my sincere thanks and deep gratitude to my supervisor Prof. Dr. Neran K. Ibrahim for her valuable support and I would like to thank her for all the discussions and the useful suggestions, and also for the patience that she always had with me. I would like also to express my grateful admiration to Prof. Dr. Thamer J. Mohammed, head of chemical engineering department. My great appreciation to all the staff of the Department of Chemical
Engineering
Department/University
of
Technology
for
providing the Facilities for this work. Also, I would like to express my deep gratitude and special thanks to Mr. Hassan Hadi , Mr. Ayad A. Mahmoud and Mr. at AL- Doura Refinery for their great help and support. Finally, thanks to all people who assisted me. Saja
I
Abstract The presence of sulfur in crude oil poses enormous challenges as regards its negative environmental and economic impact. As such, the safety of the equipment is at high risk during the processing of Al-Ahdab crude oil (3.9wt% sulfur) because of its sour nature. The present research work therefore is aimed at reducing the sulfur content of Al-Ahdab crude oil prior to processing by subjecting it to five different treating methods namely: solvent extraction, oxidation, combined oxidation/solvent extraction, oxidation assisted by adsorption and combined oxidation/extraction assisted by adsorption using relatively mild conditions. The desulfurization efficiency was investigated firstly using extraction desulfurization. Different solvents have been tested including acetonitrile, acetone and methanol. The measured sulfur content of the crude oil, obtained after 30 minutes contact time indicated that acetonitrile has the higher ability to extract sulfur compounds than acetone and methanol respectively giving desulfurization efficiency of 28.5%, using 3:1 solvent/oil ratio. The second desulfurization mode studied was oxidation, using hydrogen peroxide as an oxidation agent. The results indicate that the desulfurization efficiency increases with increasing reaction temperature (35-60 ℃), reaction time (15-60 min) and mixing speed (100-500 rpm). Also H2O2/Formic acid system showed higher performance as compared to H2O2/HCL with desulfurization efficiency 7.2% and 4.14% respectively. The best operating conditions were 60 ℃, 500 rpm and time and 60 minutes reaction time.
II
On using combined oxidation/solvent extraction with acetonitrile, the desulfurization efficiency was enhanced from 7.2% to 31.5%. The effect of operation variables on the oxidation desulfurization of crude oil using hydrogen peroxide as an oxidizing agent and activated carbon as a catalyst was studied. The sulfur content in crude oil was decreased from 3.9% to 2.622% at 60 ℃, 500 rpm and time and 60 minutes reaction time, corresponding to a desulfurization efficiency of 32.8%. The highest desulfurization efficiency achieved was 42.5%% on using combined oxidation/extraction assisted by adsorption treating method. In
this
study
an artificial
neural network
(ANNs) (consisting of two
hidden layers and twenty neuron in each layer) was used for modeling the experimental data. The output tracks the targets very well and the R2 - value was 0.999.
III
Contents Contents Subject
Page
Acknowledgement Abstract List of Abbreviations
I II XVI
List of Figures List of Tables Contents
XVII IX XIII
Chapter One: Introduction 1.1 Petroleum Hydrocarbons in the Environment 1.2 Remediation of Petroleum Hydrocarbons 1.3 Microorganisms oxidizing hydrocarbons 1.4 Problem statement 1.5 The Aim of the Present Work
1 1 3 4 5
Chapter Two: Literature Survey 2.1 Fundamental Principles of Bioremediation 2.1.1 Bioremediation 2.1.2 Bioremediation techniques 2.1.2.1 Bio augmentation 2.1.2.2 Bio stimulation 2.1.3 Types of bioremediation 2.1.3.1 In-situ Bioremediation 2.1.3.2 Ex-situ Bioremediation 2.1.4 Degradation of hydrocarbons by microbes 2.1.4 Petroleum Hydrocarbons
IV
7 7 8 8 8 9 10 11 14 17
Contents 2.2 Factors affecting bioremediation of petroleum hydrocarbons 2.2.1 Microbial factors 2.2.2. Oxygen 2.2.3. Temperature 2.2.4. Soi1 moisture 2.2.5 Nutrients 2.2.6 pH value 2.2.7 Soil structure 2.2.8. Hydrocarbon variety and concentrations 2.3 General Information on Degradation Pathway of
19 20 21 22 23 23 24 25 25 28
Petroleum Hydrocarbons 2.3.1 Degradation pathway of aliphatic hydrocarbons
28
2.3.1.1 Oxidation of hydrocarbons 2.3.1.2 Degradation pathway of aromatic hydrocarbons
28 29
Chapter Three: Materials and Methods 3. Materials and methods
31
3.1 Materials 3.1.1 Chemicals 3.1.2 Equipments 3.2 Cultures medium
31 31 32 33
3.2.1 Bushnell-Haas Medium (BHM)
33
3.2.2 Nutrient agar
33
3.2.3 Nutrient broth
33
3.2.4 Standard turbidity solution (Macfarland tube 7) 3.3 Methods
34 34
3.3.1 Sample collection
34
3.6 3.3.2 Isolation of bacteria from oil contaminated soil
35
V
Contents 3.3.3 Bacterial Identification
35
3.3.4 Inoculum preparation
36
3.3.5 Measuring of the Biodegradation of Diesel Oil 3.3.5.1 Determination of bacterial activity (biomass and
36 36
surface tension reduction) 3.3.5.2 Account the Quantitative Loss of Diesel oil 3.3.5.3 The use of FT-IR spectrum and Gas Chromatography Technique to detect the Biodegradation of Diesel Oil
37 38
3.4 Ex situ Bioremediation simulation
39
3.4.1 Measurement of soil chemical and physical properties
39
3.4.2 Preparation of contaminated soil
39
3.4.3 Bacterial consortium used for bio-augmentation, Inoculum preparation 3.5 Bio pile Tanks
40
3.5.1 Experimental design
41
3.6 Monitoring the Bioremediation experiment 3.6.1 Determination of pH, temperature, moisture of soil and the bacterial count 3.6.2 Total hydrocarbons analysis by extraction method
43 43
41
43
Chapter Four: Results and Discussion 5.1 Introduction
74
5.2 Extractive Desulfurization 5.3 Oxidation Desulfurization 5.3.1 Effect of Type of Oxidant System 5.3.2 Effect of Stirrer Speed 5.3.3 Effect of Reaction Time
75 76 76 77 78
VI
Contents 5.3.4 Effect of Reaction Temperature 5.4 Role of Extraction on the Efficiency of Oxidation Desulfurization
5.5 Oxidation / Adsorptive Desulfurization 5.5.1 Effect of Sorbent Dose 5.5.2 Effect of Stirrer Speed 5.5.3 Effect of Reaction Time 5.5.4 Effect of Reaction Temperature 5.6 Role of Activated Carbon in Adsorptive Desulfurization 5.7 Combined Desulfurization Processes 5.8 Neural Network Modeling
80 81 82 82 83 83 85 86 87 90
Chapter five: Conclusions and Future Recommendations 5.1 Conclusions 5.2 Recommendations for Future Work
References Appendices Appendix A A.1 Effect of Different Modes on Desulfurization Efficiency
Appendix B B.1 Predicted (Output) versus Target Values of ANN Structure
VII
78 79 80
Nomenclature Greek Symbols Symbol
Definition
Learning rate momentum term Dispersion bonds Hydrogen bonds Polar bonds Micron
δD δh δP µ
Unit
[-] [-] [-] [-] [-] [-]
Abbreviations Symbol
Definition
Unit
AC AHD ATF DE GAC HCO kGy LCO MSE NK PL ppm
Activated Carbon AL-Ahdeb crude, Aviation Turbine Fuel Desulfurization Efficiency Granular Activated Carbon Heavy Crude Oil Kilo Gray Light Crude Oil Mean Square Error Khana crude oil Kirkuk crude oil Part per million.
[-] [-] [-] [-] [-] [-] [-] [-] [-] [-] [-] [-]
S SRD ST STARS XRF
Residual Sulfur Straight-Run Diesel AL-Basrah crude oil Super Type II Active Reaction Sites X-ray Fluorescence
[-] [-] [-] [-] [-]
XVII
List of Figures
List of Figures Figures
Page
Fig.(1.1 ): Global Trend on Crude Oil Quality
1
Fig. (2.1): Important Classes of Sulfur-Containing Compounds in
6
Crude Oil Fig. (2.2): Structure of (a) DBTs and (b) BTs in Actual Light Oils
10
Fig.(2.3): Simplified PFD of in-Situ Electrochemical HDS of Crude
13
Fig.(2.4): General Process Flow of Extractive Desulfurization
21
Fig.(2.5): The Ideal Reaction for DBTs and BTs
30
Fig.(3.1): Schematic Diagram of the Experimental Setup
48
Fig.(3.2): A Photographic Picture of the Experimental Setup
48
Fig.( 3.3): Photographic Picture of Sulfur Content Analyzer
49
Fig.(3.4): Steps for Sample Preparing
49
Fig. (4.1): Structure of a Single Processing Node with the Sequence
61
of Processing of Information Fig(4.2): Block Diagram of a Two Hidden Layer Multilayer
62
Perceptron (MLP) Fig(4.3): Types of Neural Networks
63
Fig(4.4): The Input and Output Layer of All Operational Condition
72
IX
List of Figures
Figures
Page
Fig.(5.1): Desulfurization Efficiency versus Solvents/Oil Ratio
75
Fig.(5.2 ): Desulfurization Efficiency verses Types of Oxidant
77
System 77
Fig.( 5.4): Desulfurization Efficiency Ver
78
Fig.(5.5):
79
Fig. (5.6): Residual Sulfur Concentration versus Time,
80
Fig.(5.7): Desulfurization Efficiency Versus Different Temperature,
80
Mixing Speed 500 rpm and Time of Mixing 60 minutes Fig.( 5.8): Desulfurization Efficiency Versus Sorbent Dose at 500
82
rpm Mixing speed, 3 ml H2O2, 4 ml Formic acid and Time of Mixing 60 minutes Fig.(5.9):
83
Fig.(5.10):
84 and Mixing Speed 500 rpm
X
List of Figures
Figures
Page
Fig. (5.11 ):
Fig (5.12):
84
Desulfurization Efficiency Versus
Temperature,
86
Mixing Speed 500 rpm and Time of Mixing 60 minutes Fig (5.13): Desulfurization Efficiency versus AHD Before and
87
After Treatment with the Efficient Desulfurization Method Fig. (5.14): Effect of Oxidation Desulfurization Mode on the
88
Desulfurization Efficiency for AHD Crude Fig. (5.15): Sulfur Content for Four Types of Iraqi Crude Oil
89
Fig.(5.16 ): Regression Plot of Training Prediction Set
91
Fig.( 5.17 ): Regression Plot of Test Prediction Set
92
Fig.( 5.18): Regression Plot of All Prediction Set
92
Fig.( 5.19 ): Evolution of Function
of the
Number
Training and of Learning
Training
XI
Test
Errors
as
a
Epochs during ANN
93
List of Tables
List of Tables Tables
Page
Table (2.1): Physical Properties of Selected Sulfur-Containing
7
Compounds Table (2.2): Microwave-Promoted HDS using Iron Powder as the
12
Catalyst Table(2.3):Hildebrand Solubility Parameter Delta for Some
17
Solvents Table(2.4):"Non-Polar", "Polar Aprotic" and "Polar Protic" Solvents
20
with δP (Polar Bonds), δD (Dispersion Bonds) and δH (Hydrogen Bonds) Table(2.5):Desulfurization of Crude Oils Achieved Through Three-
41
Step Integrated Process Table( 3.1): Properties for Four Different Types of Iraqi Crudes Oil
50
Table( 3.2): Specifications of Chemicals Used in the Present Study
51
Table( 3.3): Physical Properties of the Activated Carbon
52
Table( 3.4): Ranges of Experimental Variables Studied
52
Table( 3.5): Details of the Experimental Runs
55
Table( 5.1): Different Neural Network Trained Model
90
XIII
Nomenclature
Nomenclature Symbol
Definition
[Hnmp] BF4 2,5-DMT 2-MT 4,6-DMDBT 4-MDBT ANNs API Atm BDS BMImCl BNT BP BTs C Co DBTO DBTs DMF DMSO DTAB HDS ILs MLP NHDS NMP OATS OD ODS OSC PAHs RA ULSD
N-methyl-pyrrolidonium tetraflouoroborate 2,5-dimethylthiphene 2-methylthiophene 4,6-dimethydibenzothiophene 4-methyldibenzothiophene artificial neural networks American Petroleum Institute Atmosphere Biodesulfurization 1-butyl-3-methylimidazolium chloride Benzonaphthothiophene Back Propagation Benzothiophenes Effluent concentration of the crude oil Initial concentration of of the crude oil Dibenzothiophene sulfone Dibenzothiophenes Dimethyl form amide Dimethyl sulfoxide Dodecyl Trimethyl Ammonium Bromide Hydrodesulfurization Ionic liquids Multilayer Perceptron Non- Hydrodesulfurization N-methyl pyrrolidinone Olefinic Alkylation of Thiophenic Sulfur Oxy-Desulfurization Oxidative desulfurization Organosulfur Compounds Polycyclic Aromatic Hydrocarbons Reactive Adsorption Ultra-Low Sulfur Diesel
XVI
Units
[-] [-]
Chapter One
Introduction
Chapter One Introduction 1-1 Introduction Oil is the blood of industry and also a principal item reflecting the development level of the national economy. With the ever stringent environmental protection regulations adopted, a common problem which petroleum refineries are facing around the world is that crude oil used as feedstock for refining process is becoming heavier day by day with higher sulfur content
[1]
, Figure (1-1)[2] .The
poor quality of crude oil currently can obviously result in the high sulfur contents of oil products, which can lead to corrosion, catalyst poisoning, environmental pollution and other negative consequences[3].
Figure: (1-1) Global Trend on Crude Oil Quality [2]
1
Chapter One
Introduction
Sulfur in crude oil exists in two main forms: the first is termed as “active sulfur” which can react with the metal directly; the second is “inactive sulfur” which cannot directly react with the metal. Active sulfur includes sulfur, hydrogen sulfide and mercaptan; inactive sulfur includes sulfide, carbon disulfide, thiols, thiophenes, substituted benzo- and dibenzothiophenes (BTs and DBTs), benzonaphthothiophene (BNT), and many considerably more complex molecules, in which the condensed thiophenes are the most common forms [4]. Oil refineries usually separate crude oil into several fractions and then desulfurize them separately. Capital savings can be made if most of the sulfur is removed from the crude oil before it is fractionated, [5] The treatment of high-sulfur crude oil is becoming the focus of research worldwide. The conventional equipment cannot deal with the high-sulfur crude oil during petroleum refining processes. The increasing sulfur content of crude oil also results in an increase in sulfur content in automotive gasoline, diesel fuel, and jet fuel. To meet the needs for producing clean fuels, decreasing the sulfur content of crude oil becomes an urgent task. Studying new desulfurization technology and raising the efficiency of desulfurization processes are the keys to bringing more profits to the oil refining companies [2]. At present, oil desulfurization technology can be broadly divided into two categories: hydrodesulfurization (HDS) and non-HDS (NHDS). HDS is a kind of technology where H2S is formed when hydrogen adsorbed on catalyst at high temperature and high pressure reacts with sulfur. HDS is one kind of more mature technology, but there several shortcomings such as high running costs and needing a lot of hydrogen, all these increased the cost of oil significantly. Non-HDS technology does not use hydrogen source and is in line with the requirements of deep desulfurization, so researchers focus on non-HDS
2
Chapter One
Introduction
technology. In the last decade, oxidative desulfurization (ODS) has been studied much. ODS is a kind of technology using oxidants oxidizing organic sulfur to strong polarity matters, and the reaction products can be separated by absorption or extraction. ODS is operated at atmospheric pressure and the temperature is below 100oC, so it has mild reaction conditions, no needs for hydrogen, and pressure reactor and special operating technology.[9] ODS has high selectivity, so that the sulfur compounds (BT, DBT, etc) which are removed with difficultly in HDS can be easily removed by oxidation. So ODS is a promising desulfurization technology because of its low production cost. Sulfur content in oil can be reduced through other organic solvent extraction, thereby reducing the impact on the environment. Various studies on the ODS process have reported the use of differing oxidizing agents, such as H2O2 oxidation method, organic oxidant method, photochemical oxidation method, as well as those involving the use of plasma or ultrasound.[9] Much effort has been devoted to developing techniques that can reduce or remove such refractory sulfur compounds by Oxidation/Extraction, Adsorption and Biodesulfurization . Desulfurization by selective adsorption is considered one of the most promising ultra - deep desulfurization methods. Adsorption is the most common HDS alternative method used to achieve ultra clean fuels.
[6]
Adsorption is a mass
transfer process wherein molecules in a free phase become bound to a surface by intermolecular forces [7]. It is often employed to remove trace impurities, such as the removal of trace amounts of aromatics from aliphatics
[8]
. Adsorption is
effective in separation processes involving low sorbate concentrations, and thus potential exists for removal of refractory sulfur compounds in transportation fuels. [10]
3
Chapter One
Introduction
2-1 Scope of the Present Work The present study focuses on: 1. Investigating the effectiveness of five different modes of desulfurization processes namely: solvent extraction, oxidation, combined oxidation/ solvent extraction, oxidation assisted by adsorption and combined oxidation/extraction assisted by adsorption for desulfurization of Iraqi crude oil (AL-Ahda crude oil
sulfur content
2. Studying the effect of the main operating variables: contact time, mixing speed, solvent/oil ratio and temperature on the desulfurization efficiency. 3. Modeling the experimental data obtained using the ANN model.
4
Chapter Two
Literature Survey
Chapter Two Literature Survey 2.1 Introduction A common problem facing refineries around the world is that crude oils are becoming heavier, with higher sulfur contents, which results in higher sulfur levels in both straight-run and secondarily processed diesel oil. Therefore, it is preferable to reduce the sulfur content of crude oil before the oil is refined. So petroleum quality faces severe challenge, which demands urgent research and development of processing techniques to produce high quality oil products. Various methods were suggested for the desulfurization of the oil and refinery streams.
These
strategies
include
Hydrodesulfurization,
extractive
desulfurization, oxidative desulfurization, biodesulfurization, alkylation-based desulfurization, and chlorinolysis-based desulfurization. Despite the variety of methods reported, few of the strategies are viable for the desulfurization of heavy oil. This is mainly due to the properties of the heavy oil, such as high sulfur content, high viscosity, high boiling point, and refractory nature (resistant to heat) of the sulfur compounds.[11]
2.2 Sulfur Compounds in Crude Oil Sulfur is the most abundant element in petroleum after carbon and hydrogen. The average sulfur content varies from 0.03 to 7.89 wt% in crude oil. The sulfur compounds can be found in two forms: inorganic and organic. Inorganic sulfur, such as elemental sulfur, H2S and pyrite can be present in dissolved or
5
Chapter Two
Literature Survey
suspended form. Organic sulfur compounds such as thiols, sulfides, and thiophenic compounds represent the main source of sulfur found in crude oil. Some of the important classes of organic sulfur compounds are shown in Figure (2 -1).[11]
Figure (2 -1): Important Classes of Sulfur-Containing Compounds in Crude Oil (R = alkyl) [11] Crude oils with higher viscosities and higher densities usually contain higher amounts of more complex sulfur compounds. The aliphatic acyclic sulfides (thioethers) and cyclic sulfides (thiolanes) are easy to remove during a hydrodesulfurization process or by thermal treatment. On the other hand, sulfur contained in aromatic rings, such as thiophene and its benzologs (e.g. benzothiophene, dibenzothiophene, benzonaphthothiophene) are more resistant to sulfur removal by hydrodesulfurization.
[12]
Other sulfur compounds may be
present in crude oil but in very limited quantities like sulfoxides, sulfons, sulfonic acid, and alkyl sulfonates.
[13,14]
The concentration and nature of the
sulfur-containing compounds change over the boiling range. The amount of sulfur in a distillation fraction increases with an increase in boiling range, with
6
Chapter Two
Literature Survey
the heaviest fraction containing the most sulfur. The sulfur compounds become more refractory with increasing boiling point, as the dominant compound class changes from thiols, sulfides, and thiophene in the naphtha to substituted benzothiophenic compounds in the distillate Table (2-1). In the vacuum gas oil and vacuum residue, the sulfur is contained mainly in compounds of the dibenzothiophene family. [11] Table (2-1): Physical Properties of Selected Sulfur-Containing Compounds [11] Compound
1-Ethanethiol(ethylmercaptan)
Normal boiling point C
35
Melting (kg m- 3) -144.4
839.1
Dimethyl sulfide
37.3
- 98.3
848.3
1-Propanethiol(propylmercaptan)
67
-113.3
841.1
Thiophene
84.2
-38.2
1064.9
Diethyl sulfide
92.1
-103.8
836.2
1-Butanethiol (butyl mercaptan)
98.4
-115.7
833.7
Dimethyl disulfide
109.7
-84.7
1062.5
Tetrahydrothiophene (thiolane)
121.1
-96.2
998.7
Dipropyl sulfide
142.4
-102.5
837.7
Thiophenol
168.7
-14.8
1076.6
Dibutyl sulfide
185
-79.7
838.6
Benzothiophene (thianaphthene)
221
32
1148.4
Not reported in Dibutyl disulfide
226
reference
Dibenzothiophene
332
99
7
938.3
Not reported in reference
Chapter Two 2.3
Literature Survey
Desulfurization Technologies
2.3.1 Hydrodesulfurization (HDS) 2.3.1.1 Conventional HDS: Hydrodesulfurization is one of the catalytic desulfurization processes, which aims at turning organic sulfur compounds into H2S using H2 as the reactant in the presence of metal catalysts operating at high temperature and pressure. The resultant hydrogen sulfide is then removed from the system. This method is widely used in oil industry since 1955. However, the HDS process features a complicated procedure, high production cost and high materials consumption. With an increasing ratio of heavy crude oil supply to the oil refinery, the sulfur content in crude slate is growing, which leads to a shortened catalyst life at the refinery. Furthermore, the HDS process needs more H2, so the production cost will increase a lot.
[2]
Berger et al.
[15]
showed that the active catalyst material
commonly consists of combinations of Co - Mo, or Ni – Mo, or nickel and tungsten. HDS catalysts slowly lose activity with using, and must be removed and replaced every two to three years. [16] Deep desulfurization via HDS would require either increasing reactor residence time or carrying out reactions in harsher conditions (higher temperature, pressure). [17] elevated pressures ranging from 30 to 130 atmospheres of absolute pressure. Temperatures in the reactor typically range from (315 to 370 oC). At these temperatures, some or all of the feed may vaporize, depending on the boiling range of the feed and the pressure in the unit.[16] The specific conditions depend on the degree of desulfurization required and the nature of the sulfur compounds in the feed. Alphatic sulfur compounds are
8
Chapter Two
Literature Survey
very reactive and can be removed completely during HDS (Equations 2.1 to 2.3), while sulfur contained in thiophenic rings is more difficult to remove [11]: Thiols: R-SH+H2 R-H+ H2S
(2.1)
Sulfides: R1-S-R2 +2H2 R1-H+ R2-H+ H2S
(2.2)
Disulfides: R1-S-S-R2+3H2 R1-H+ R2-H+ 2H2S
(2.3)
The low-boiling crude oil fractions contain mainly the aliphatic organosulfur compounds: mercaptans, sulfides, and disulfides. They are very reactive in conventional hydrotreating processes and they can easily be completely removed from the fuel. Other processes like Merox can be applied to extract mercaptans and disulfides from gasoline and light refinery streams.
[18]
All versions of the
Merox process are characterized by the catalytic oxidation of mercaptans (RSH) to disulfides (RSSR) in an alkaline (basic) environment. The overall reaction is: 2RSH + 1/2 O2 RSSR + H2O However Dishun et.al
[16]
(2.4)
found that the HDS is limited in treating
benzothiophenes (BTs) and dibenzothiophenes (DBTs), especially DBTs having alkyl substituents on their 4 and / or 6 positions, as presented in Figure (2-2). The production of light oil, with very low levels of sulfur-containing compounds, therefore requires inevitably the application of severe operating conditions and the use of especially active catalysts.
9
Chapter Two
Literature Survey
Figure (2-2): Structure of (a) DBTs and (b) BTs in Actual Light Oils [16] 2.3.1.2 Advanced HDS Akzo Nobel introduced in 1998, highly active Co-Mo and Ni-Mo catalysts referred to as STARS (Super Type II Active Reaction Sites). Under usual HDS operating conditions, these catalysts are claimed to desulfurize refinery streams down to 2–5 ppm of sulfur and to significantly reduce polyaromatic content and improve the cetane number and density of diesel fuels. Both Co-Mo and Ni-Mo catalysts can be used for deep desulfurization but their efficiency is determined by the feedstock properties. The Co-Mo STARS catalysts are preferable for streams with relatively high sulfur levels of 100–500 ppm and perform better than Ni-Mo catalysts at low pressure. In contrast, the Ni-Mo STARS catalysts are especially suitable for fuels with low sulfur levels (below 100 ppm) at high pressure. [18] In summary HDS is industrially employed for upgrading heavy oil, its effectiveness is undermined by the following properties of heavy oils: [11] a) High metal content, which causes deposit formation and catalyst deactivation. b) Coking and fouling tendency, this results in catalyst deactivation. c) Molecular size, which limits access to smaller catalyst pores.
10
Chapter Two
Literature Survey
d) Steric protection of thiophenic sulfur, making adsorption for HDS difficult. 2.3.1.3 Hydrodesulfurization (HDS) Assisted by Ultrasonic Energy Ultrasonic energy can be used for catalytic HDS of thiophene. There is only one preliminary report about ultrasound desulfurization of thiophene - waterethanol mixture employing Ni/Al2O3 or Ni/ZnO catalysts at low temperature and atmospheric pressure.
[19]
The combination of ultrasound and catalyst results in
water decomposition to provide hydrogen for thiophene desulfurization. The reported desulfurization level is about 30 – 40 mol% of thiophene conversion. [18] 2.3.1.4 Microwave-Promoted Desulfurization Microwave irradiation is attracting increasing attention as a tool for facilitating chemical reactions. It is often possible to shorten reaction times dramatically as well as improve product conversion. Leadbeater and Khan
[20]
have probed the
effect of microwave irradiation on desulfurization of crude oil and dibenzothiophene. A range of catalysts have been screened. It was found that it is possible to perform hydrodesulfurization reactions using microwave heating in conjunction with iron powder as a catalyst. The effects of hydrogen pressure, reaction temperature, reaction time, and catalyst source have been studied. Microwave energy can also be used as the power for regeneration of the HDS catalyst. Integrating microwave, catalysis and hydrogenation together in the desulfurization process is more efficient than the traditional technology of desulfurization as shown in Table (2-2). Microwave inducement can improve the effect of chemical desulfurization and better desulfurizing results are gained.
11
Chapter Two
Literature Survey
Table (2-2) Microwave-Promoted HDS using Iron Powder as the Catalyst [20] Level of
Test No.
Feed oil
Reaction conditions
1
Dibenzothiophene
50 psi H2
200 C f
20
i
8
2
Crude oil
50 psi H2
200 C f
20
i
25
3
Crude oil
20 psi H2
200 C f
20
i
10
4
Crude oil
No H2
5
Crude oil
50 psi H2
250 C for 20 min
24
6
Crude oil
50 psi H2
200 C f
27
7
Crude oil
50 psi H2
200 C
200 C f
desulfurization, %
20
i
30 f
No reaction detected
i
110
7
2.3.1.5 Electrochemical Desulfurization Hammad et.al
[10]
present a work on electrochemical in-situ hydrogen
generation route which is an alternative route that eliminates the need for an external hydrogen source for HDS. The process involves parallel reactors for batch operation. For continuous operation, parallel reactors can also be employed. For batch operation, diagram is shown in Figure (2-3).
12
Chapter Two
Literature Survey
Figure (2-3) Simplified PFD of in-Situ Electrochemical HDS of Crude Fractions [10] 13
Chapter Two
Literature Survey
During the study, the electrochemical desulfurization reaction was carried out with the addition of various electrolytes, Case A: 0.25 (M) H2SO4, Case B: combination of 0.1 wt% Dodecyl Trimethyl Ammonium Bromide (DTAB, C15H34N.Br) and 0.25 (M) H2SO4, Case C: standalone 0.1 wt% (DTAB). The results demonstrated that despite the presence of a high concentration of H 2. The % sulfur removal is not as high as observed i ―C e A ‖. It was concluded that the total maximum % sulfur removal (95%) is reached after ~ 4 hours of batch operation. As the reaction progressed with time, the % sulfur removal decreased as the gas phase sulfur products moved into the liquid phase by other reverse reactions. The advantages of in-situ HDS is that it is carried out in milder conditions, at lower temperature (~250 °C) and pressure (~40 atm) with lower hydrogen partial pressure. The process also has the ability to accommodate various types of low cost electrocatalysts for targeting the poly-aromatic molecules during the electrocatalytic reactions, thereby reducing the total cost.
2.3.2 Extractive Desulfurization 2.3.2.1 Solvent Extraction Liquid-liquid extraction is an important kind of separation method that is based on the distribution of chemicals between two different liquid phases.
[22]
Amongst the various methods of separation, solvent extraction, or liquid-liquid extraction, is considered to be the most versatile and popular method of separation. [23]. 2.3.2.2 Solvents Classification Solvent is a liquid substance capable of dissolving other substances (solutes) without chemical changes
[24]
, resulting in a solution that is soluble in a certain 14
Chapter Two
Literature Survey
volume of solvent at a specified temperature.
[25]
. In order for a solvent to be
able to accomplish this task it must overcome the forces that bind the solute (the substance being dissolved) together.[26] Molecules are held together by electrostatic attractions called Van der Waals forces. However the forces causing molecules to stick together are much weaker than chemical bonds. Chemical bonds are the forces that bind the atoms of a molecule together. Although Van der Waals forces are weak they affect the properties of solvents and only solvents with Van der Waals forces similar to those of the solute will be able to pull the molecules of the solute apart so that it will dissolve. [26] Lowery and Richardson classified solvents into two categories: polar and nonpolar. [25] Solvent ability to dissolve another substance is determined by ability of its molecular structures (like dissolves like) and the types of molecular structures of the solvents are as follows: [24] Polar protic solvents: A polar protic molecule consists of a polar group OH and a non-polar tail. The structure may be represented by a formula ROH. Polar protic solvents dissolve other substances with polar protic molecular structure and these solvents are miscible with water (hydrophilic). Examples of polar protic solvents: water (H-OH), acetic acid (CH3CO-OH) methanol (CH3-OH), ethanol (CH3CH2-OH), npropanol (CH3CH2CH2-OH), n-butanol (CH3CH2CH2CH2-OH). Dipolar aprotic solvents: Dipolar aprotic molecules possess a large bond dipole moment (a measure of polarity of a molecule chemical bond). They 15
Chapter Two
Literature Survey
do not contain OH group. Examples of dipolar aprotic solvents are acetone (CH3)2C=O), ethyl acetate (CH3CO2CH2CH3), dimethyl sulfoxide (CH3)2SO ), acetonitrile (CH3CN), dimethylformamide ( (CH3)2NC(O)H). Non-polar solvents: Electric charge in the molecules of non-polar solvents is evenly distributed; therefore the molecules have low dielectric constant. Non-polar solvents are hydrophobic (immiscible with water) and these solvents are lipophilic as they dissolve non-polar substances such as oils, fats, greases. Examples of non-polar solvents include carbon tetrachloride (CCl4), benzene (C6H6), and diethyl ether (CH3CH2OCH2CH3), hexane (CH3(CH2)4CH3), methylene chloride (CH2Cl2). Generally, the dielectric constant of the solvent provides a rough measure of a solvent's polarity. The strong polarity of water is indicated, at 20 °C, by a dielectric constant of 80.10. Solvents with a dielectric constant of less than 15 are generally considered to be nonpolar. [26] The polarity, dipole moment, polarizability and hydrogen bonding of a solvent determine what type of compounds is able to dissolve and with what other solvents or liquid compounds is miscible. Al- Malki
[21]
indicated that the extraction efficiency depends on the
lve ’
polarity, which has to be sufficient to remove sulfur compounds. Examples of polar solvents include those with high values of the Hildebrand solubility parameter deltas as shown in Table (2-3). Liquids with a delta higher than about 22 have been successfully used to extract these compounds. The Hildebrand solubility parameter (δ) p vide
u e ic l e i
e
f he deg ee
interaction between materials, and can be a good indication of solubility.
16
f
Chapter Two
Literature Survey
The Hildebrand solubility parameter is the square root of the cohesive energy density: δ = (∆Hυ – RT / Vm)½
(2.5)
Where δ = Cohesive energy density
∆Hυ = Heat of vaporization R = Gas constant T = Temperature Vm = Molar volume. The cohesive energy density is the amount of energy needed to completely remove unit volume of molecules from their neighbors to infinite separation, which is equal to the heat of vaporization divided by molar volume. In order for a material to dissolve, these same interactions need to be overcome as the molecules are separated from each other and surrounded by the solvent. Materials with similar solubility parameters will be able to interact with each other, resulting in solvation, miscibility or swelling. Table (2-3): Hildebrand Solubility Parameter Delta for Some Solvents [16,21 , 27] Solvent
Hildebrand
Acetone
19.7
Ethanol
26.2
Methanol
29.7
DMSO
26.4
Acetonitrile 30 DMF
24.7
17
Chapter Two
Literature Survey
Polarity however is not the only criterion for the selection of suitable solvents. Methanol, for example has sufficient polarity, but its density, 0.79 g/cm3, is about the same as that of typical light diesel oil. Other properties such as boiling point, freezing point, and surface tension need to be considered carefully to evaluate the potential for separation and recovery of the solvent for recycling and reuse [16,21, 27]. 2.3.2.3 Factors Affecting Dissolving Power [26] As mentioned earlier, a solvent's ability to dissolve a solute is dependent upon how similar its electrostatic forces are to those of the solute. There are three component forces that make up these Van der Waals forces. These component forces are dispersion forces, polar forces and hydrogen bonding forces. - Dispersion Forces
[26]
Dispersion forces are caused by the temporary attractions generated by the dynamic nature of electron orbits. Because electrons vary their paths as they circle the nucleus, pockets of electrostatic force are created on the surface of a molecule whenever a nucleus is temporarily exposed by the dispersed electrons. Dispersion forces are size dependent; the larger the molecule, the stronger the bond between molecules. -Polar Forces
[26]
Polar forces are dependent upon molecular characteristics such as atomic composition, geometric shape and size. Some molecules have areas of their surface with a permanent electrostatic force which causes them to attract other molecules. Molecules that display these constant electrostatic attractions are
18
Chapter Two
Literature Survey
called polar molecules. Polar molecules vary in the strength of their electrostatic forces because molecules vary in the characteristics that generate these forces. - Hydrogen Bonding Forces Hydrogen bonding forces a strongest intermolecular force
[28]
and specialized
electrostatic force that is usually created by the electrical attraction between the hydrogen of one molecule with the oxygen of another. When the single electron orbiting a hydrogen atom is pulled away by another atom in a molecule, a strong electromagnetic attraction is created by the exposed hydrogen proton. The hydrogen bonding force of a solvent increases as the number of exposed hydrogen atoms in a molecule increases. [26] Table (2-4) [25] shows that the intuitions from "non-polar", "polar aprotic" and "polar protic" are put numerically – the "polar" molecules have higher levels of δP (polar bonds)
d he p
ic
lve
h ve highe level
f δH(hydrogen
bonds). Because numerical values are used, comparisons can be made rationally by comparing numbers. So acetonitrile is much more polar than acetone but slightly less hydrogen bonding.
19
Chapter Two
Literature Survey
Table (2-4): "Non-Polar", "Polar Aprotic" and "Polar Protic" Solvents with δP (Polar Bonds), δD (Dispersion Bonds) and δH (Hydrogen Bonds). [25] δH Solvent
Chemical formula
δ
r
δP P l r
Hydrogen bonding
Non-polar solvent Benzene
C6H6
18.4
0
2
Hexane
C6H14
14.9
0
0
Toluene
C6H5-CH3
18
1.4
2
Polar aprotic solvents Acetone
Ch3-C(=O)-CH3
15.5
10.4
7
Acetonitrile
CH3-CN
15.3
18
6.1
Dimethyl form amide
H-C(=O)N(CH3)2
17.4
13.7
11.3
CH3-S(=O)-CH3
18.4
16.4
10.2
(DMF) Dimethyl sulfoxide (DMSO) Polar protic solvents Ethanol
CH3-CH2-OH
15.8
8.8
19.4
Methanol
CH3-OH
14.7
12.3
22.3
Formic acid
H-C(=O)OH
14.6
10
14
2.3.2.4 Applications of Extractive Desulfurization Babich and Moulijn
[18]
explain that the extractive desulfurization is based on
the fact that organosulfur compounds are more soluble than hydrocarbons in an appropriate solvent. The most attractive feature of the extractive desulfurization is the applicability at low temperature and low pressure Moreover the process
20
Chapter Two
Literature Survey
does not change the chemical structure of the fuel oil components. The general process flow of extractive desulfurization is shown in Figure (2-4) [11]
Figure (2-4): General Process Flow of Extractive Desulfurization [11] Hamad et.al
[36]
employed solvent extraction and hydrotreating for the
desulfurization of crude oil. A high sulfur content crude oil feed stream was treated by mixing one or more selected solvents with a sulfur-containing crude oil feed stream for a predetermined period of time, allowing the mixture to separate and form a sulfur-rich solvent-containing liquid phase and a crude oil phase of substantially lowered sulfur content, withdrawing the sulfur-rich stream and regenerating the solvent. The remaining sulfur-rich stream was hydrotreated to remove the sulfur-containing compounds to provide a hydrotreated low sulfur content stream. The hydrotreated stream was then mixed with the separated crude oil phase to provide a treated crude oil product stream of substantially reduced sulfur content and without significant volume loss. Javadli and De Klerk
[11]
reported that the efficiency of extractive
desulfurization is limited by the solubility of the organosulfur compounds in the 21
Chapter Two
Literature Survey
solvent. An appropriate solvent selection is very important for efficient desulfurization. This is usually achieved as Babich and Moulijn
[18]
reported by
preparing a ''solvent cocktail'' such as acetone–ethanol or a tetraethylene glycol– methoxytri glycol mixture .The preparation of such a‘ solvent cocktail ’is rather difficult and intrinsically non-efficient since its composition depends strongly on the spectrum of the organosulfur compounds present in the feed stream. Different types of solvents have been tried such as acetone, ethanol, and polyethyleneglycols, which resulted in 50 – 90% desulfurization depending on the number of extraction cycles of the process. [18] The liquid - liquid extraction technique using water - soluble polar solvents such as Dimethyl sulfoxide (DMSO), Dimethyl formamide (DMF), methanol and acetonitrile) were usually used. The former two solvents have a high extractability for sulfones but also have a high boiling point at 573K. This is close to the boiling point of the sulfones, thus creating difficulty in separation and reuse for further extraction. [27, 30]
Shiraishi et al. [30] have used acetonitrile as an extraction solvent, since it has a relatively low boiling point (355K) and can be easily separated from the sulfones by distillation. Martinie et al. [27] preferred the use of methanol and acetonitrile as the extraction solvent due to their polarity, volatility and low cost. So when methanol and acetonitrile were contacted with light oil, a large quantity of aromatics was extracted simultaneously with the sulfones.
22
Chapter Two
Literature Survey
2.3.2.5 Ionic Liquid Extraction Ionic liquids can be considered as green solvents due to their very low vapor pressure and wide range of applications with unique physical and chemical
properties.
The potential of ionic liquids has been recognized
worldwide.[31] The extractive desulfurization of fuels by ionic liquids (ILs) as opposed to traditional organic solvents is an interesting alternative to provide ultra clean fuels. Ideal ionic liquids have a high distribution coefficient for sulfur compounds, a low cross solubility for the hydrocarbons, a low viscosity, and fast phase separation rate after mixing and extraction. [11] Ionic liquids systems revealed a high desulfurizing activity toward gasoline; where the ionic liquid BMImCu2Cl3 extracted 23 % of sulfur compounds, whereas BMImBF 4 extracted no more than 11 %.[32] In 2001 Bösmann et al
[33]
published a first paper describing deep
desulfurization of real diesel fuel by extraction with chloroaluminate ionic liquids where 80% of the sulfur components were removed successfully in a five -
ge ex
ci
Huang et al.
60 ◦C. [34]
applied ILs, obtained by reaction of 1-butyl-3-
methylimidazolium chloride (BMImCl) with anhydrous powdered CuCl, containing CuCl2– Сu2Cl3–, and Cu3Cl4– anions that are
resistant to moisture
and air, for desulfurization of a model fuel. Wang Jianlong et al.[32]
used Brönsted acidic ionic liquid of N-methyl-
pyrrolidonium tetraflouoroborate ([Hnmp] BF4) as extracting agent and catalyst to reduce the sulfur in model oil. The solution of dibenzothiophene (DBT) dissolved in n-octane was used as the model of diesel fuels containing organosulfur compounds, and the 30% (mass fraction) aqueous solution of H 2O2 23
Chapter Two
Literature Survey
was used as oxidant. The experimental results showed that the hydroxyl free radicals produced in BF4- H2O2 system could oxidize DBT to form corresponding sulfone, then sulfur was removed from model oil. In the operation, under the conditions of molar ratio of H 2O2 to S equal to 3, reaction temperature 60 C, reaction time 1 hour and volume ratio of model oil to IL equal to 1, 1550mg L-1 of S in model oil was removed completely. The more the initial concentration of DBT, the more difficult the complete removal of S is. Moreover, the ionic liquid [Hnmp]BF4 could be recycled 7 times without a significant decrease in the rate of sulfur removal. Seeberger and Jess
[35]
present that sulfur compounds can be separated from
sulfur-loaded ionic liquids by addition of water. The distribution coefficient of sulfur compounds in ionic liquids decreases to almost zero if enough water is added to the system. The sulfur compounds, together with some light hydrocarbons that were extracted, can then dissolved or form a second liquid phase in the water. This strategy is more efficient if the sulfur compounds are oxidized. There are no reports on the extractive desulfurization to heavy oil as Javadli and De Klerk
[11]
presented. This is not surprising considering the difficulties
even with straight run distillate desulfurization.
2.3.3 Adsorptive Desulfurization Sulfur compounds are known to be slightly polar than the hydrocarbons of similar nature. The exploitation of the polarity factor has been the subject of numerous investigations for processes based on oxidation/extraction and adsorption. A typical adsorption process is expected to offer selective removal of sulfur compounds, to operate at normal temperatures and pressures with ease of
24
Chapter Two
Literature Survey
operation/ease of process control and economically achieve near total elimination of sulfur compounds from transportation fuels. At the same time, ease of regeneration with minimum requirement of chemical and energy is also expected from the point of view of commercial viability. A wide variety of materials starting from activated carbon (AC), silica-based sorbents, zeolites and metal exchanged/impregnated AC/zeolites/mesoporous materials have been reported for adsorption.[37] Desulfurization by adsorption depends on the ability of a solid sorbent to selectively adsorb organosulfur compounds from the oil. The efficiency of this method depends on the properties of the sorbent material: selectivity to organosulfur compounds relative to hydrocarbons, adsorption capacity, durability, and regenerability. There are two approaches that can be used for adsorptive desulfurization: [11,18] (a) Physical adsorption, where the sulfur compounds are not chemically altered by the separation. The energy that is required for regeneration depends on the strength of the adsorption, but being a physisorption only, it is not energetically very demanding. (b) Reactive adsorption, which involves a chemical reaction between organosulfur compounds and solid sorbent surface. Sulfur is usually attached to the sorbent as a sulfide. Sulfur is usually removed as H2S, SOX or elemental sulfur, depending on the process and the nature of feedstock. Salem
[37]
investigated the desulfurization of naphtha using AC, zeolite 5A and
zeolite 13X. The sulfur compounds were mainly mercaptan, disulfide and
25
Chapter Two
Literature Survey
thiophene. A capacity of approximately 0.534 mg/g (AC), 0.123 mg/g (5A) and 0.77 mg/g (13X) was obtained. Mikhail et al.
[38]
used sorbents such as charcoal, petroleum coke, cement kiln
dust and clays for the removal of dimethyl disulfide from cyclohexane. Capacities up to 15.8 mg/g were reported for charcoal while for others it was substantially less. Song and Ma
[39]
, Song
[40]
, and Velu et al.
[41,42]
reported a series of
experimental results on the desulfurization of transportation fuels such as gasoline, diesel, jet fuel and model diesel mixtures with initial sulfur content of about 300 ppmw, using undisclosed metal supported sorbents. The capacity reported, however, was not high and breakthrough capacity of about 1 cm3/g was observed for the model diesel system. McKinley and Angelici [43] studied sulfur removal on silver-loaded mesoporous materials for model fuel comprising DBT and 4,6-DMDBT. The reported sorption capacity was not high. Hernandez-Maldonado and Yang
[44]
showed
that Cu–Y and Ag–Y zeolites have good capacity for thiophene sulfur removal from benzene and n-octane mixtures. Yang and co-workers further reported data on these adsorbents in combination with AC and using gasoline and commercial diesel [45,46]. AC was used
―Gu d Bed‖ although it was found to have some
capacity for sulfur compounds, especially for 4,6-DMDBT. With the layered bed configuration, a breakthrough capacity up to 34 cm3/g was obtained for commercial fuels.
[47,48]
Capacities up to 7.5 mg sulfur per gram of adsorbent
were reported on various modified Y-zeolites such as Ni–Y, Cu–Y, Ce–Y. A study on different commercial adsorbents and comparison with Cu–Y was also reported. [49]
26
Chapter Two
Literature Survey
Alhamed and Bamufleh [50] investigated sulfur removal from model diesel fuel using granular activated carbon (GAC) from dates, stones activated by ZnCl2. A model diesel fuel composed of nC10H34 and dibenzothiophene (DBT) as sulfur containing compound was used. It was found that more than 86% of DBT was adsorbed in the first 3 hours which gradually increased to 92.6% in 48 hours and no more sulfur was removed thereafter. The efficiency of sulfur removal by GAC decreased when applied to commercial diesel fuel. Linear regression of the experimental data obtained was able to predict the critical pore diameter for DBT adsorption to be 0.8 nm and it was concluded that activated carbon with higher average pore size has a higher adsorption capacity. The high capacity in these studies was believed to be due to π-complexation and activated carbon (AC) actually seems to play some role not just in the sorption of aromatics but also sulfur compounds including 4,6-DMDBT.
[77]
Activated carbon is a unique material because of the way it is filled with "holes" (voids, spaces, sites and pores) whatever the size of molecules. The special property about these holes is that, although they are spaces of zero electron density, these pores possess intense Van der Waals forces (from the near proximity of carbon atoms) and these are responsible for the adsorption process. [51]
2.3.4 Oxidative Desulfurization The properties of sulfur and carbon are similar in some ways, e.g., the electronegativity of sulfur is very similar to that of carbon. Therefore, the sulfur– carbon bond is relatively non-polar and sulfur containing compounds exhibit properties quite similar to their corresponding organic compounds. This is the reason why the solubility of sulfur containing compounds and hydrocarbons in 27
Chapter Two
Literature Survey
polar and non-polar solvents are nearly identical. However, if these sulfur containing compounds that are present in fuels could be oxidized to their corresponding sulfoxides or sulfones their solubility in polar solvents would increase with an increase in their polarity. [56] Generally, desulfurization by selective oxidation involves two main steps: first: the sulfur containing compounds present in fuel are oxidizeled to the corresponding sulfoxides and sulfones by an oxidant, and then these sulfoxides and sulfones are removed from the fuel by extraction, adsorption, or distillation. [35,53]
Dishun et.al
[16]
reported that in many cases the process improves the fuel
quality. The advantages of the oxidative desulfurization process can be summarized as follows: [54] -
Does not use hydrogen to produce ultra-low sulfur diesel (ULSD).
-
Mild operation conditions.
-
Complementary chemistry to hydrodesulfurization
-
Uses conventional reaction and separation refinery equipment. Collins et al [55] reviewed another feature of oxidative desulfurization (ODS)
which is that the refractory sulfur compounds in ODS are easily converted by oxidation. 2.3.4.1 Autoxidation Pasiuk-Bronikowska et al. [56] present a special case of autoxidation (oxidation by atmospheric oxygen, i.e. oxygen in air) involving a catalyst or oxygen
28
Chapter Two
Literature Survey
carrier. The results showed that autoxidation
proceeds readily at a low
temperature and typical conditions for selective autoxidation are less than 200 C and near atmospheric pressure. While during the autoxidation of heavy oil some sulfur is typically removed as SO2. Most of the sulfur compounds are converted into sulfoxides and sulfones, which can be separated from treated crude oil by a second step.[11] During the autoxidation of crude oil, some hydrocarbon molecules are oxidized too. Insoluble oxidation products are formed that appear as gums and sediments. The formation of gums and sediments are more when oxidizing oil sand-derived bitumen is compared with lighter oil fractions
[57]
but one of the key challenges
that remain in the application of autoxidation for the ODS of heavy oil is the avoidance of free radical addition reactions. These reactions result in a significant viscosity increase upgrading.
Nevertheless,
[58]
it
, which complicates transport and downstream was
demonstrated
that
around
46–47%
desulfurization of Cold Lake bitumen is possible with autoxidation followed by water extraction [11]. 2.3.4.2 Chemical Oxidation Guth and Diaz
[59]
and Guth et al.[60] used nitrogen dioxides followed by
extraction with methanol to remove both sulfur and nitrogen compounds from petroleum stocks. Tam and Kittell
[61]
describe a process for purifying hydrocarbon aqueous oils
containing both heteroatom sulfur and heteroatom nitrogen compound impurities, such as shale oils, by first reacting the oil with an oxidizing gas containing nitrogen oxides and then extracting the oxidized oil with solvents in
29
Chapter Two
Literature Survey
two stages (amines and formic acid). Patrick et al.[62] used the oxidationextraction process, operating at ambient pressure and low temperature (typically 0-30 C), with nitric acid or nitrogen oxides as oxidants, and one of several polar o
solvents for extraction. Zannikos et al.
[63]
reported that a combination of oxidation with solvent
extraction is capable of removing up to 90% of the sulfur compounds in petroleum fractions at acceptable liquid yield. Oxidants can donate oxygen atoms to the sulfur in mercaptans (thiols), sulfides, disulfides and thiophenes to form sulfoxides or sulfones as illustrated in Figure (2-5) all of these oxidized sulfurcontaining compounds are orders of magnitude more soluble in non-miscible solvents than their unoxidized counterparts. [16] The second step of this process is the removal of the oxidized compounds by contacting the distillate with a selective extraction solvent. [63,64]
Figure (2 -5): The Ideal Reaction for DBTs and BTs [16] The sulfur levels were markedly reduced from 4720 µg/g to 70 µg/g and the API gravity as well as the cetane number was improved.
30
Chapter Two
Literature Survey
In 1996 Petro Star Inc. combined conversion and extraction to remove sulfur from diesel fuel.
[52]
First, the fuel was mixed with H2O2/acetic acid
(peroxyacetic acid) and the oxidative reaction took place below 100 °C under atmospheric pressure. This was followed by a liquid/liquid extraction to obtain a fuel with low sulfur and an extract with high sulfur. The low sulfur fuel might require additional treatment. The extraction solvent was removed from the extract for reuse and the concentrated extract was made available for further processing to remove sulfur. The extraction solvent is also important for the success of the oxidation/extraction process. The reactivity of the different sulfurcontaining compounds including methyl phenyl sulfide, thiophenol, diphenyl sulfide, 4-methyldibenzothiophene
(4-MDBT),
dibenzothiophene
(DBT),
benzothiophene (BT), 2-methylthiophene (2-MT), 2,5-dimethylthiphene (2,5DMT), and thiophene was investigated for selective oxidation using hydrogenperoxide and formic acid by Otsuki et.al [65] and the oxidative reactivities of the sulfur-containing compounds was also investigated. Te et.al
[66]
studied the reactivity of DBT, 4-MDBT, and 4,6-DMDBT using
toluene solutions as the model diesel in a polyoxometalate/H 2O2 system. The oxidation reactivities decreased according to DBT > 4-MDBT > 4,6-DMDBT, and the same reactivity trend was found for HDS. The apparent activation energies of DBT, 4-MDBT, and 4,6-DMDBT oxidation were 53.8, 56.0, and 58.7 kJ/mol, respectively. The results indicated that DBT oxidation may be achieved under mild reaction conditions and it is easy to increase the reaction temperature or reaction time to achieve high conversions, even for the least reactive 4,6-DMDBT, which showed a decrease in reactivity compared with DBT as more methyl substituents are present at the 4 and 6 positions on the DBT rings. 31
Chapter Two Zongxuan et.al
Literature Survey [67]
indicated that the sulfones in the oxidized fuel oils can be
removed by a polar extractant and the sulfur level of a prehydrotreated diesel can be lowered from a few hundred µg/g to 0.1 µg/g after oxidation and subsequent extraction whereas the sulfur level of a straight-run diesel can be decreased from 6000 to 30µg/g after oxidation and extraction. De Filippis and Scarsella
[68]
used a hydrogen peroxide and formic acid
oxidizing system to study the influence of the solvent on the oxidation rate of the sulfur-containing compounds in the organic phase. The results indicate that heterocyclic sulfur-containing compounds such as benzo- and dibenzothiophene have different kinetic processes compared with thiols and sulfides. The aromaticity of organic solvents has a considerable influence on oxidation rates. Stanciulescu and Lkura [69] indicated that the addition of ethanol to acetonitrile as an additive improve the oxidative desulfurization of diesel from 56.67 % to 60.18%. The experimental results established that tert-butyl hydroperoxide is better oxidant than hydrogen peroxide while acetonitrile and N-methyl pyrrolidinone (NMP) are found to have almost the same efficiency as solvents. The effect of mixing on the oxidation process using hydrogen peroxide as oxidant and NMP as solvent is very effective; it reduces the reaction time from 6 hrs to 1 hr and also improves the desulfurization of diesel to77.65%. 2.3.4.3 Catalytic Oxidation Ramírez-Verduzco et al.[70] investigated dibenzothiophene removal by an oxidation-extraction. The experiments were carried out to observe the role that the solvent plays during the process, as well as the oxidizing agent and catalyst. The oxidation was carried out with hydrogen peroxide in the presence 32
Chapter Two
Literature Survey
of a catalyst of tungsten supported zirconia (WOx-ZrO2).A dibenzothiophene and n-hexadecane model mixture was employed to simulate a diesel fuel. Methanol, ethanol, acetonitrile and γ-butyrolactone were used as extraction lve
. Dibe z hi phe e w
e
ved
with respect to other solvents. The highe
e efficie ly by γ-butyrolactone e c ivi y w
chivied whe
γ-
butyrolactone was used during DBT oxidation with and without catalyst. When oxidation was carried out without catalyst, the oxidant behavior of the mixture could be explained in terms of the dissociation of the hydrogen peroxide to produce strong oxidant species like perhydroxyl ions (HO 2-) by the influence of the aprotic solvents. When a catalyst was used during the oxidation, there was an additional oxidation contribution through the formation of surface peroxo-metal intermediates (W-O-O-H). Rao et.al
[71]
studied oxidative desulfurization of hydrodesulferized diesel,
hydrodesulferized diesel doped with dibenzothiophene (DBT) and isooctane doped with DBT by using H2O2, tert-butyl hydroperoxide as oxidants, metal acetylacetonates like vanadyl
acetylacetonate, manganese acetylacetonate,
molybdenum acetylacetonate, iron acetylacetonate and sodium tungstate as catalysts for oxidation of sulfur compounds to sulfones/sulfoxides followed by solvent extraction with acetonitrile or N-methyl pyrrolidinone (NMP). Vanadyl acetylacetonate was found to be the most effective catalyst among the various catalysts studied with both H2O2 and tert-butyl hydroperoxide oxidants. A maximum
oxidative
desulfurization
efficiency
of
77.65
%
for
the
hydrodesulferized diesel (540 ppmw initial sulfur content) was obtained. The results indicated that while it is easier to oxidize and remove DBT sulfur, it is relatively difficult to oxidize and remove sulfur compounds like 4,6-dimethyl
33
Chapter Two
Literature Survey
dibenzothiophene and other similar hindered alkyl dibenzothiophene derivatives present in diesel using such oxidative desulfurization system. Yu et al [72] investigated the oxidative desulfurization of commercial diesel fuel with hydrogen peroxide in the presence of activated carbon and formic acid. The effect of aqueous pH on the catalytic activities of the activated carbon was studied. It was found that the oxidation of DBT is enhanced when the aqueous pH is less than 2, and the addition of formic acid can promote the oxidation. The effect of carbon surface chemistry on DBT adsorption and catalytic activity was also investigated. Adsorption of DBT showed a strong dependence on carboxylic group content. The oxidative removal of DBT increased as the surface carbonyl group content increased. The results showed that, on using activated carbon adsorption, 98% of sulfur could be removed from the diesel oil without any negative effects on the fuel quality. Garcia-Gutierrez et al. [73] investigated the reactivity of Mo/γ -Al2O3 catalysts in the oxidative desulfurization process of diesel fuel using hydrogen peroxide as the oxidizing reagent. The catalysts were prepared by equilibrium adsorption using several molybdenum precursors and alumina with different acidity values. The effects of reaction time, reaction temperature, nature of solvent, concentration of solvent and hydrogen peroxide, and content of molybdenum and phosphate in the catalysts were investigated. The results showed that the activity toward sulfur elimination depends mainly on the presence of hepta- and octamolybdates species on the supported catalyst and the use of a polar aprotic solvent. Likewise, the presence of phosphate markedly increase sulfur elimination. It was possible to reduce the sulfur level in diesel fuel from about 320 to less than 10 µ g/g at 60 oC under atmospheric pressure.
34
Chapter Two Liu et.al
[74]
Literature Survey
studied the oxidative desulfurization (ODS) of Azeri crude oil
under the electric field. The desulfurization process was carried out in the electric desalting unit, which performs the functions of desulfurization, desalting and dehydration at the same time. The optimum conditions for desulfurization of crude oil were identified. The influence of the four main factors, including the desulfurizer dosage, the oil bath temperature, the distilled water injection rate, and the demulsifier dosage, on the desulfurization rate was studied while the effect of oxidative desulfurization under the electric field was compared with that of the single oxidative desulfurization method. The test results showed that the desulfurization reaction taking place under the electric field is better than the traditional method. The desulfurization rate of crude oil could reach 77.06% at 115 ℃
de ulfu ize d
ge f 200 μg/g
d
de ul ifie d
ge f 50 μg/g.
Also the test results showed that the density and viscosity of crude oil decrease with the removal of sulfur compounds while other basic properties of crude oil are little affected after desulfurization process. 2.3.4.4 Photochemical Oxidation It was reported that photochemical oxidation has a high efficiency and requires mild reaction conditions. The method involves two steps: first, sulfur compounds are transferred from the oil into a polar solvent and then the transfer is followed by photo oxidation or photo decomposition under UV irradiation. The oxidation chemistry is similar to the other oxidation methods, but instead of thermal energy, energy is supplied by light. Various methods have been developed for different types of light oil and organosulfur compounds such as thiophene, benzothiophene, and dibenzothiophene.
[75,76]
although good sulfur removal
around (90%) was achieved during experiments with model light oils. [11]
35
Chapter Two
Literature Survey
2.3.4.5 Ultrasound Oxidation Ultrasonic wave can become a very useful tool in the oil desulfurization industry. Ultrasonic wave combined with other technologies can reduce the temperature, the consumption of power and solvent, the amount of waste discharge and the production cost, which can also increase the efficiency of recycling and product purity. On the other hand, this process does not cause environmental pollution to promise a rosy future.[2] Mei et.al
[78]
, studied ultrasound assisted oxidative desulfurization to obtain
ultra-low sulfur diesel fuel. Using appropriate oxidants and catalysts with the assistance of ultrasound irradiation, model compounds such as dibenzothiophene can be quantitatively oxidized in minutes. For diesel fuels containing various levels of sulfur content, and through the use of catalytic oxidation and ultrasonication followed by solvent extraction, removal efficiency of sulfurbearing compounds can reach or exceed 99% in a short contact time at ambient temperature
and
atmospheric
pressure.
The
conversion
of
DBT
(dibenzothiophene) to DBTO (dibenzothiophene sulfone) with and without the use of ultrasound was examined. It was found that the oxidation of DBT to DBTO reachs over 85 and 95% within 1 and 3 min of ultrasonication, respectively. In 7 min, DBT was completely oxidized to DBTO. In comparison, the conversion of DBT to DBTO in the absence of ultrasound was only 21% in 1 min and reached barely above 80% in 7 min, which is still less than the conversion with 1 min sonication. Flores et.al
[79]
used oxidation assisted by ultrasound for the desulfurization of
fuel oils. The effects of hydrogen peroxide concentration studied, fuel oil to aqueous solution volumetric ratio, and type of catalyst. The studied catalysts 36
Chapter Two
Literature Survey
were FeCl3 and CuSO4. They also studied the ability of acetonitrile to extract sulfur compounds from the fuel oil. It was found for the heavy fuel oil, the sulfur reduction due to acetonitrile washing is 18.4% and it was found the active catalyst is FeCl3 getting around 70% of sulfur reduction, and only around 5% of sulfur reduction is obtained when CuSO4 is used. On the other hand, the sulfur reduction in diesel enhanced when the H2O2 concentration is 3 wt% , but further increase in the H2O2 concentration to 6 wt%
does not improve the sulfur
reduction. Redha
[29]
used oxidative desulfurization assisted by 35 kHz frequency
ultrasonic irradiation to reduce the sulfur content of East Baghdad crude oil. The experimental results revealed that the highest removal efficiency is 76.45%, obtained at 60 oC , 80 W/cm2 sonication power intensity in time applied for 12 minutes and a ratio of 100 ppm H2O2/100 ppm formic acid. In another study ultrasound-assisted oxidative desulfurization followed by extraction was applied to various diesel fuels in the presence of hydrogen peroxide with a transition metal complex and quaternary ammonium salts as catalyst. It was found that desulfurization exceeds 95% in a short period of time under ambient conditions.
[80]
This is in agreement with an earlier work
conducted by Collins et.al [81] who showed that the oxidation reaction could take place without the use of ultrasound, but at a much lower rate. The best run indicated that the oxidation of diesel containing 0.253 wt% S followed by solvent extraction with methanol produces desulfurized diesel with sulfur content of 0.085%, which corresponds to overall sulfur removal of 66.4% after 2.5 h reaction. The sulfur removal efficiency could be further improved to reach
37
Chapter Two
Literature Survey
97.8% by increasing the reaction time to 4 h and using silica adsorption instead of solvent extraction to remove oxidizing sulfur species from diesel. 2.3.4.6 Microwave Oxidation A recent study has been carried out by Wang et al.[1] on the simultaneous desulfurization and demetalization of crude oil by electric desalting based on the oxidation of alkyl thiophene and nickel compounds with the compound of Chitosan Schiff Base under the condition of microwave irradiation. Chitosan is the product of N-deacetylation of widely existing chitin in shell fish and cell walls of fungi or plants. The influence of Chitosan Schiff Base compound dose, initial temperature, and microwave time on desulfurization and
demetalization was investigated. Crude
oil samples
were prepared by
dissolving 0.6 g of DBT and 0.1 g of nickel porphyrin in a mixture of 40 g of toluene and 59.3 g of hexane. 100 mL of a crude oil sample was combined with a given dose of Chitosan Schiff Base solution in the microwave reactor and after it was irradiated by microwave, removal experiments were carried out in the electric desalting apparatus. By using the optimized conditions for microwave irradiation , up to 56% of sulfur and 82% of nickel removal rates were achieved for model compounds in crude oil samples. Under the same conditions but without microwave the removal efficiency was lower than 56% for sulfur model compound and lower than 82% comparison with technology, the
for nickel
conventional
porphyrin
in
crude
oil samples. In
desulfurization
and
demetalization
proposed microwave irradiation process can
accomplish
the simultaneous removal of sulfur and nickel compounds from crude oil under relatively mild conditions.
38
Chapter Two
Literature Survey
2.3.5 Biodesulfurization (BDS) Biodesulfurization has been applied since the 1980s. In this process, the sulfur is transformed into sulfides and H2S by biological reduction, and the elemental sulfur can be removed via the process of biological oxidation. [82] This process takes place at low temperatures and pressure in the presence of microorganisms that are capable of metabolizing sulfur compounds. It is possible to desulfurize crude oil directly by selecting appropriate microbial species.[83] There are two potential benefits to BDS such as lower capital and operation costs. It has been reported that BDS requires approximately two times less capital and 15% less operating cost in comparison with traditional HDS.
[84]
Also there are two
disadvantages associated with this method. Firstly, this process is not very efficient. Secondly, one kind of bacteria can only remove one or few sulfides. [85] Other factors may limit the application of this technology; the process is sensitive to environmental conditions such as sterilization, temperature, and residence time of the biocatalyst.[86] Leshchev
[87]
invented the hydrogenation-bacterial catalytic process. This
method involves two steps: firstly, hydrogenation of the crude oil is carried out in order to remove unstable organic sulfides contained in the crude oil, and then an artificial culture of organic sulfur compounds with selective bacteria named Rhodococcus Rhodocrous is used in the desulfurization process. In the biological oxidation process, organic sulfides (such as dibenzothiophene series) are subjected to C—S bond cleavage reaction and turned into inorganic sulfides. Hence this method can remove sulfur compounds from oil products efficiently. Agarwal and Sharma
[88]
present a study in biodesulfurization (BDS) of light
crude oil (LCO) and heavy crude oil (HCO) using Pantoea agglomerans 39
Chapter Two
Literature Survey
D23W3 resulted in 61.40% removal of sulfur for LCO, whereas HCO showed 63.29% S removal under similar conditions. The use of P. agglomerans D23W3 under anaerobic conditions showed marginally better results than those under aerobic conditions. Bhatia and Sharma[89] used a newly isolated strain Pantoea agglomerans D23W3 in biodesulfurization of dibenzothiophene from light crude oil, heavy crude oil, diesel, HDS diesel and aviation turbine fuel (ATF). It was found that P. agglomerans D23W3 could degrade 93% of the 100 ppm DBT within 24 hours of culture. In addition P. agglomerans D23W3 could also desulfurize 4,6dimethyl DBT and benzothiophene which are among the most difficult DBT derivatives to be removed by HDS. Further, adapted cells of P. agglomerans D23W3 were found to remove 26.38–71.42% of sulfur from different petroleum oils. Therefore, P. agglomerans D23W3 have a potential for the BDS of the petroleum oils as they concluded. The BDS-OD-RA method is an integrated process in three-step for desulfurization of high-sulfur crude oil by BDS. Desulfurization of low-sulfur crude oil by BDS needs aerobic conditions, combined with oxidation and adsorptive desulfurization as shown in Table:(2-5)
[90]
. So this combined
technology for crude oil desulfurization is an important development trend.
40
Chapter Two
Literature Survey
Table:( 2-5) Desulfurization of Crude Oils Achieved Through Three-Step Integrated Process [90] Sample to be treated
HCO
LCO
HCO
LCO
S content, %
1.88
0.378
1.88
0.378
First-step treatment
BDS (AN)
BDS (AN)
BDS (AO)
BDS (AO)
S content after the first step, %
0.58
0.138
0.69
0.15
Desulfurization rate, %
69.14
63.40
63.29
61.4
Second-step treatment
OD
OD
OD
OD
S content after the second step %
0.11
0.07
0.23
0.08
Desulfurization rate, %
94.15
81.48
87.76
78.84
Third-step treatment
RA
RA
RA
RA
S content after the third step, %
0.11
0.053
0.10
0.0
Desulfurization rate, %
94.15
85.9
94.6
94.3
Notes: HCO: heavy crude oil, LCO: light crude oil, BDS: biodesulfurization, OD: oxydesulfurization, RA: reactive adsorption, AO: aerobic, AN: anaerobic.
Adegunlola, G. A et.al
[92]
investigated microbial desulfurization of crude oil
using Aspergillus Flavus where sulfur content of crude oil is exposed to microorganisms that can specifically break carbon sulfur bond, thereby releasing sulfur in a water soluble, organic form. Rhodococcus sp. and Arthrobacter sulphureus are bacteria used in desulfurization of fossil fuel . They are used for reducing the sulfur content of diesel samples. Aspergillus 41
Chapter Two
Literature Survey
flavus was used to desulfurize crude oil under three different conditions. These include different time durations, temperatures and concentrations of immobilized spores. The effect of different time duration on sulfur removal from crude oil using A. flavus was studied. 100ml of crude oil samples were treated for one, two, three and seven days. The amounts of sulfur removed were 27.2%, 45.2%, 90.4% and 91.7% respectively. The effect of different temperatures on sulfur removal from crude oil was also investigated. When the temperature was i c e ed
40C
d 45C, A. flavus was able to reduce the sulfur level of the oil
by 55.3% and 10.5% respectively. It can be deduced from this result that the higher the temperature, the lower the amount of sulfur removed. At higher temperature, enzymatic activity and catalytic property of the organism can be disrupted.
2.3.6 Chlorinolysis-Based Desulfurization Chlorinolysis involves the scission of C-S and S-S bonds through the action of chlorine (Equations 2.4 and 2.5) [91] R-S-R´+CL2R-S-CL+ R´-CL
(2.6)
R-S-S-R´+CL2R-S-CL+R´-S-CL
(2.7)
Where R and R' represent hydrocarbon groups The process is pe f
ed
l w e pe
u e (25- 0) C and near atmospheric
pressure; it requires a short residence time (5-120) min, good mixing of oil and chlorine gas and equipment having adequate corrosion resistance to chlorine. At moderate temperature and in the presence of water, chlorinolysis can be followed by hydrolysis and oxidation of the sulfur to produce sulfates.
42
[91]
A 3:10
Chapter Two
Literature Survey
volumetric ratio of water to oil works best. This is followed by aqueous and caustic washes to remove the sulfur and chlorine containing by-products. Around 75–90% of total sulfur can be removed in an hour. The handling, cost, and pollution probability by chlorine gas may be limitations for practical application also there is a safety risk associated with such operation and the volume of chlorine required is considerable. [11]
2.3.7 Alkylation-Based Desulfurization 2.3.7.1 C-alkylation When the boiling temperature of organosulfur compounds is shifted to a higher value, they can be removed from light fractions by distillation and concentrated in the heavy boiling part of the refinery streams. [93,94] This technology has been tested with thiophenic sulfur compounds at small scale, and it was commercially applied to light oil at large scale as the Olefinic Alkylation of Thiophenic Sulfur (OATS) process was developed by British Petroleum.[95] It is impractical to apply this type of desulfurization technology to broad distillation cuts, or heavy distillation cuts. In both instances separation by distillation is difficult due to boiling point overlap and the need to remove the alkylated sulfur compounds as bottom product. This technology is consequently not suitable for the desulfurization of heavy oil. [11] 2.3.7.2 S-alkylation Thiophenic compounds react with iodomethane (CH3I) in the presence of silver tetrafluoroborate (Ag-BF4) to produce S-methylatedsulfonium salts. [96,97] These alkylated sulfur compounds can then be removed from the oil as precipitates,
43
Chapter Two
Literature Survey
thereby effectively desulfurizing the oil. It does not require separation by distillation as in the case of C-alkylation, which simplifies the separation. [11] However, alkylation takes place competitively with aromatic hydrocarbons, eroding its applicability to oils that are aromatic rich. Since heavy oils tend to be aromatic, this technology as Javadli and De Klerk [11] reported is not suitable for the desulfurization of heavy oils.
2.3.8 Radiation Desulfurization Basfar and Mohamed [98] made a study on radiation-induced desulfurization for four types of Arabian crude oils (heavy, medium, light and extra light) and straight-run diesel (SRD) over the range of 10–200 kGy (Kilo Gray). The objective of the presented investigation was to develop an improved process for the desulfurization of crude oils and SRD by radiation. Moreover, oxidation of sulfur compounds due to oil-irradiation makes oil safer for transportation and more useful for further refining. Results showed that gamma radiation processing at absorbed doses up to 200 kGy without further treatment is not sufficient for desulfurization. In conclusion, the results indicate that most of the organically bound sulfur and/or elemental sulfur and hydrogen sulfide still exist even after exposure to 200 kGy. They recommended that sulfur removal from crude oil and SRD can
be achieved
by
combining
gamma-irradiation
with
other
physical/chemical processes (i.e. liquid-liquid extraction, adsorption and oxidation), which may be capable of removing considerable levels of sulfur compounds in the investigated products.
44
Chapter Two
Literature Survey
2.4 Concluding Remarks: At present, the commercial methods for desulfurization of petroleum products have their own deficiencies. It is necessary to improve the desulfurization process to maximize the sulfur removal efficiency. The desulfurization efficiency and economic benefits should be compatible, and the existing facilities should be fully utilized to provide quick returns. And it is also necessary to support the development of appropriate technologies that can reduce low-value products, increase high-value products, improve economic efficiency, and enhance the competitive edge of the enterprise. The difficulties in crude oil desulfurization are aggravated by the existence of polycyclic aromatic hydrocarbons (PAHs), nitrogenous compounds and H2S. Global crude oil production has been nearly reaching the limit that technology can hardy break through. By building new devices or optimizing the related technologies, the product yield can be improved and the materials consumption during oil refining can be reduced, which will bring about significant economic returns.[2] The following specific observations are made, based on the review of desulfurization literature and the applicability of different desulfurization strategies for heavy oil:
The extractive desulfurization can be easily integrated into the refinery
and the equipment used is rather conventional without special requirements.
Although ionic liquids have high distribution coefficient for model sulfur
compounds, such as dibenzothiophene, in model mixtures, the distribution coefficient in real straight run distillate is rather low. In other words, ionic liquids are not ideal solvents for extractive desulfurization of real straight run distillates. In heavy oil the situation becomes worse. The efficiency of an 45
Chapter Two
Literature Survey
extraction process with ionic liquids increases if the organosulfur compounds are previously oxidized to corresponding sulfoxides and sulfones, since oxidized sulfur compounds have much higher distribution coefficient.
[11]
so
ionic liquid extraction is not feasible as desulfurization method for heavy oil. Biodesulfurization may lead to successful desulfurization, but there are technical obstacles related to the refractory nature of the sulfur molecules that must be metabolized, high viscosity, and the complexity of the heavy oil.
Some chemicals that is more expensive. This disqualifies alkylation, chlorinolysis, and many of the chemical oxidation processes for desulfurization.
Autoxidation (oxidation with air as oxidant) is a viable desulfurization strategy. Autoxidation itself leads to little desulfurization and it must be used in combination with a sulfur removal step.
46
Chapter Three
Experimental Work
Chapter Three Experimental Work 3.1 Introduction The aim of the present work is the desulfurization of AL-Ahdab crude oil (̊API=25.8, sulfur content = 3.9%). Five different modes of desulfurization processes namely: solvent extraction, oxidation, combined oxidation/ solvent extraction,
oxidation
assisted
by
adsorption
and
combined
oxidation/extraction assisted by adsorption using relatively mild condition have been investigated. The influence of: solvent/oil ratio, type of solvent, mixing speed, temperature and time on the desulfurization efficiency was also examined.
3.2 Experimental Setup The experiments were carried out in a laboratory scale setup consists of 500 ml three necked round bottom flask. The flask is connected to a reflux condenser. A thermometer is placed in one of the two side necks and the third neck is used for sample withdrawal. The flask is submerged in a temperature controlled water bath. A hot plate magnetic stirrer made in Korea, Model LMS1003, with speed control from 60-1500 rpm. Electric Supply 220V, 50/60 Hz. A separatory funnel is used for the separation of solvent from the bulk of crude oil. Schematic diagram and photographic picture of the experimental setup are present in Figure (3.1) and (3.2) respectively.
47
Chapter Three
Experimental Work
Figure :(3-1) Schematic Diagram of the Experimental Setup
Figure :(3-2 )A Photographic Picture of the Experimental Setup
48
Chapter Three
Experimental Work
3.3 Sulfur Content Analysis The sulfur content in crude oil feedstock was measured with X-ray fluorescence according to ASTM D-4296 using SELFA-2800 analyzer made by Horiba company, USA, which is shown in Figure(3-3)
Figure :(3-3) Photographic Picture of Sulfur Content Analyzer This devise is assisted with Spectro Membrane consist of a thin-film sample support by substance attached to a frame that serves as a carrier, sample preparing includes three steps:
A
Step 1.Around 10 ml of AHD crude oil is poured in a XRF sample cup. Step 2.SpectroMembrane carrier frame is then placed over XRF sample cup. Step 3.Snap-on ring or sleeve is pressed over Spectro Membrane carrier frame and the carrier frame is torn away. Figure :(3-4)A&B shows sample preparing steps. These tests were carried out in AL-Dura Refinery Analysis Laboratory.
Figure :(3-4)Steps for Sample Preparing 49
B
Chapter Three
Experimental Work
3.3 Materials 3.3.1 Feedstock As compared to the available Iraqi crude oils, and as shown in Table (3-1), AL-Ahdab crude oil (AHD) is the heaviest crude with the highest sulfur content; therefor this crude is used as a feedstock in this study. Table :( 3-1) Properties for Four Different Types of Iraqi Crudes Oil [99]
Types of Iraqi Crudes oil
Property
AHD
PL
ST
API
25.8
33.6
30.3
44
Sulfur content, wt.%
3.9
2.8
3.16
0.61
Carbon residue wt.%
8
4.79
6.17
0.73
Asphaltenes wt.%
4.79
1.6
3.3
0.09
Density
0.896
0.859
0.8777
Pour Point C
Below -30 Below -30 Below -30
NK
0.8082 -12
Nickel ppm
22.5
11.6
12.9
10.7
Vanadium ppm
80.4
30.3
43.1
8.5
AHD: AL-Ahdab crude, PL: Kirkuk crude oil, ST: AL-Basrah crude oil NK: Khana crude oil
50
Chapter Three
Experimental Work
3.3.2 Chemicals The chemicals used in this study were of analytical grade, Specifications, properties and suppliers of these materials are listed in Table (3-2) Table :(3-2) Specifications of Chemicals Used in the Present Study Substance
Formula
Molecular weight
Specific gravity, at 25o C
Purity/ concentration
Supplier
Purpose
Adjustment Formic acid
HCOOH
46
1.2
99.99%
Fluka
of aqueous phase pH
Hydrogen
H2O2
34.01
1.11
30 w.w%
Fluka
oxidant
Methanol
CH3OH
32.04
0.791
>99.9%
Aldrich
solvent
Acetone
CH3COCH3
58.08
0.787
99.8%
Lab-scan
solvent
Acetonitrile
CH3CN
41.05
0.79
99.8%
Baker
solvent
peroxide
Adjustment Acetic acid
HCL
60.05
1.052
99.9%
Baker
of aqueous phase pH
Distilled water
H2 O
18.01
1.0
51
Almansour
diluent
Chapter Three
Experimental Work
For adsorption mode, industrial grade activated carbon (AC) with mean particle size of 1mm was used. Before use AC was dried in an oven at 110oC for 4 hours to remove any moisture. The physical properties of the AC are presented in Table (3-3). Measurements were conducted in the Catalyst Laboratory of Petroleum Research and Development Center using automated surface area and porosity analyzer. Table (3-3): Physical Properties of the Activated Carbon Test
Results
Surface area (m2/g)
702
Pore volume (cm3/g)
0.59
Bulk density (g/cm3)
0.72
Particle density (g/cm3)
1.91
3.4 Ranges of Experimental Variables Studied The ranges of the studied operating variables are given in Table (3-4): Table (3-4): Ranges of Experimental Variables Studied Variable
Range
Mixing speed
100-500 rpm
Mixing time
15-240 min
Temperature
30-60 C
Solvent/crude ratio
1:1 to 3:1
Formic acid
4 ml
Additives type
formic acid and acetic acid
Sorbent dose (AC)
0.2- 1 g
52
Chapter Three
Experimental Work
3.5 Experimental Procedure: The experimental runs were performed using five modes of desulfurization as follows: 1. Solvent Extraction 2. Oxidation 3. Combined Oxidation/ solvent extraction. 4. Oxidation /adsorption.
5. Combined oxidation / extraction assisted by adsorption 3.5.1Desulfurization Using Solvent Extraction Mode Selectivity is the first property that should be examined for the applicability of solvents in a separation process. From this point of view, the desirable solvent would extract the maximum desired components (i.e., sulfur-containing compounds) and minimum of undesired compounds (i.e., different hydrocarbon compounds present in the crude oil). The sulfur-containing compounds are more polar than other accompanying hydrocarbon compounds and, hence, they can be extracted by using a polar solvent. Based on the above requirements, three types of solvent candidates, including acetonitrile, acetone, and methanol were tested. The desulfurization process is as follows: 1. A mixture consisting of 20 ml of AHD crude oil and 20 or 40 or 60 ml of tested solvent including acetonitrile, acetone, and methanol is prepared and Mixed for 30 minutes at 30 ̊ C and 300rpm mixing speed. 2. After the solvent extraction treatment, the liquid mixture is transferred to 1000 ml glass separator funnel where the mixture is separated into a first
53
Chapter Three
Experimental Work
phase of crude oil of reduced sulfur content and a solvent phase containing dissolved sulfur compounds, in where separation of two liquids of marked densities difference, separation will occur by settling. 3. Total sulfur concentration in oil phase is determined using sulfur analyzer by x-ray fluorescence (XRF).
3.5.2 Desulfurization Using Oxidation Mode 1. An aqueous solution consisting of 3ml hydrogen peroxide, 4ml of formic acid or acetic acid , and 5ml distilled water is prepared.[72] 2. This solution is mixed with 100 ml of AHD crude oil. 3. After a certain reaction period, the mixing is stopped; contents allowed cooling to room temperature ≈30 ̊C, then the sulfur content is measured.
3.5.3 Oxidation Assisted by Adsorption A typical procedure is as follows: 1. A mixture of 100 ml AHD crude oil , 3ml hydrogen peroxide, 4ml formic acid, 5ml distilled water and 0.7g activated carbon is stirred at 500 rpm and 600C for 60 min. [72] 2. The contents are allowed to cool to room temperature. Then, the reaction mixture is transferred to a filtration paper to separate the activated carbon particles from the reaction mixture for which the sulfur content is measured. Full details of the experimental runs are given in Table (3-5).
54
Chapter Three
Experimental Work Table (3-5): Details of the Experimental Runs
Extraction
Mode:
Experimental Run No.
Variable
Constant Parameters
Time :
30 ̊ ,300 rpm, S/O: 1:1
1
10
and acetone
2
20
as solvent
3
30
4
40
5
50 Type of solvent:
30 min, 300 rpm,
6
Acetone
30 ̊ C and S/O: 1:1
7
Acetonitrile
8
Methanol S/O
30 min, 300 rpm
9
1:1
10
2:1
30 ̊ C and acetone or acetonitrile or methanol as solvent
11
3:1
Oxidation
Mode:
Time (h) 12
1
13
2
14
3
15
4
16
Time (min) 15
17
30
18
45
19
60
55
60 ̊
, 200 rpm
and H2O2/Formic acid
60 ̊ ,500 rpm and H2O2/Formic acid
Chapter Three
Experimental Work Experimental run
Variable
Oxidation
Mode:
Temperature ̊ 20
60
21
50
22
40
23
30 Speed rpm
24
100
25
200
26
300
27
400
28
500
Oxidation + Adsorption
Mode:
Time (h) 29
1
30
2
31
3
32
4 Time ( min)
33
15
34
30
35
45
36
60
56
Constant parameters 60 min , 500 rpm and H2O2/Formic acid
60 min , 60 ̊ and H2O2/Formic acid
30 ̊ ,200rpm , 0.7g and H2O2/Formic acid
60 ̊ ,500rpm , 0.7g and H2O2/Formic acid
Chapter Three
Experimental Work
Oxidation + Adsorption
Mode:
Experimental run
Variable
Constant parameters
Temperature ̊ C
60 min, 0.7g , 500 rpm and H2O2/Formic acid
37
30
38
40
39
50
40
60 Speed rpm
41
100
42
200
43
300
44
400
45
500
Adsorption+ Extraction
Oxidation+
Mode:
Weight of AC g 46
0
47
0.2
48
0.5
49
0.7
50
1
51
______
57
60 min, 0.7g ,60 ̊ and H2O2/Formic acid
60 min, 60 ̊ ,500 rpm and H2O2/Formic acid
60 min, 60 ̊ ,500 rpm , H2O2/Formic acid, S/O: 3:1 and acetonitrile as solvent
3.6
Experimental Work Experimental run
Variable
Constant parameters
52
______
60 min, 0.7g 60 ̊ , 500 rpm, H2O2/Formic acid , S/O: 3:1 and acetonitrile as solvent
Extraction
Oxidation+ Adsorption+
Mode:
Chapter Three
Determination of the Desulfurization Efficiency (DE %)
The desulfurization efficiency is calculated as the ratio of sulfur removed to that initially present in crude oil.
DE (%) =
Co – C
* 100
Co
58
…………. (3-1)
Chapter Four
Implementation of ANN
Chapter Four Implementation of ANN 4-1 Introduction Since the 1940s, artificial neural networks (ANNs) have been used in various applications in engineering and science. ANNs are generally the software systems that imitate the neural networks of the human brain
[100]
. ANNs can be
applied successfully to learning, relating, classification, generalization, and characterization and optimization functions. Because ANNs have the ability to work with incomplete data, and possess error tolerance, they can easily form models for complex problems. Moreover, they can be cheaper, faster and more adaptable than traditional methods. The prediction has been by far the second most popular neural network applications in chemical engineering (the first being process control)
[101]
besides the high costs of the experimental work, In
addition the major processes in the chemical engineering are unfortunately nonlinear. Therefore a model based on some experimental results can be developed to predict the required data instead of doing more experiments. ANN creates a connection between input and output variables and keeps the underlying complexity of the process inside the system.[102]
4-2 Artificial Neural Networks (ANNs) ANN is an especially efficient algorithm to approximate any function by learning the relationships between input and output vectors. These algorithms can learn from the experiments, are tolerant to partial and noisy data input, and can be improved continuously as more operational data are made available
[103]
.
The ANNs are able to deal with non- linear problems, and once trained can 59
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perform prediction and generalization rapidly. Especially it is desired to have the minimum difference between the predicted and observed (actual) outputs. Neural networks resemble the human brain in the following two ways: 1. A neural network acquires knowledge through learning. 2. A neural network's knowledge is stored within inter-neuron connection strengths known as synaptic weights. [104] ANNs are composed of many simple elements called neurons (which are the foundation of neural network and in many scientific and engineering applications, these nodes are frequently called a processing elements)[101] that are interconnected by links and act like axons to determine an empirical relationship between the inputs and outputs of a given system. Artificial neural networks are trained by adjusting these input weights (connection weights), so that the calculated outputs may be approximated by the desired values. [102] The neural nets learn to recognize the patterns of the data sets during the training process. Neural nets teach themselves the patterns of the data set letting the analyst to perform more interesting flexible work in a changing environment. Although neural network may take some time to learn a sudden drastic change, it is excellent to adapt constantly changing information. However the programmed systems are constrained by the designed situation. Neural networks build informative models whereas the more conventional models fail to do so. Performance of neural networks is at least as good as classical statistical modeling, and even better in most cases. [102]
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Figure (4-1) shows the basic structure of a single processing unit in an ANN, which will be referred to as a node and is analogous in concept to a single neuron in the human brain. The flow of information is as follows. [105]
Figure: (4-1) Structure of a Single Processing Node with the Sequence of Processing of Information. [105] A node receives one or more input signals yi, which may come from other nodes in the net or from some other source such as an external input of data. Each input is adjusted according to the value wi,j (not shown in Figure (4-1)), which is called a weight and that usually calls a coefficient in the ANN. These weights are conceptually similar to the synaptic strength between two connected neurons in the human brain. The weighted signals to the node are summed, and the resulting signal a, called the activation, is sent to a transfer function , g, which can be any type of mathematical function but is usually taken to be a simple bounded differentiable function such as the sigmoid function as shown in equation (4.1).[105]
F net
1 ------------------------------------ (4.1) 1 e net
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The logistic function is also known as the sigmoid function. It has a continuous derivative, which allows it to be used in backpropagation.
[107]
In the feed
forward (the information flows forward from the inputs) types of ANNs, a group of nodes called the input layer receives a signal from some external source. In general, this input layer does not adjust the signal unless it needs scaling. Another group of nodes, called the output layer, returns the signals to the external environment. The remaining nodes in the network are called hidden nodes because they do not receive signals from or send signals to an external source or location. The hidden nodes may be grouped into one or more hidden layers. Each of the arcs between two nodes (the lines between the circles in Figure: (4-2)) has a weight associated with it. Figure: (4-2) shows a layered network in which the layers are fully connected from one layer to the next (input to hidden, hidden to hidden, hidden to output). Although this type of connectivity is frequently used, other patterns of connectivity are possible.[105] Figure (4-3) illustrates the types of neural networks.
Figure: (4-2) Block Diagram of a Two Hidden Layer Multilayer Perceptron (MLP) 62
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Types of neural networks
Feedforword neural network
Recurrent network
Stochastic neural networks
Modular neural networks
Radial basis function (RBF)
Single-layer perceptron Multi-layer perceptron
Kohonen selforganizing network
Figure (4-3) Types of Neural Networks [115] 4-3 Feedforward Neural Network The feedforward neural networks are the first and arguably simplest type of artificial neural networks devised. As described earlier in this network, the information moves in only one direction, forward, from the input nodes, through the hidden nodes (if any) and to the output nodes. There are no cycles or loops in the network. [107] The most common neural network model is the multilayer perceptron (MLP). This type of neural network is known as a supervised network because it requires a desired output in order to learn. The goal of this type of network is to create a model that correctly maps the input to the output using historical data so that the model can then be used to produce the output when the desired output is unknown. This class of networks consists of multiple layers of computational
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units, usually interconnected in a feed-forward way. Each neuron in one layer has directed connections to the neurons of the subsequent layer. In many applications the units of these networks apply a sigmoid function as an activation function. Multi-layer networks use a variety of learning techniques, the most popular being back-propagation. Here the output values are compared with the correct answer to compute the value of some predefined error-function. By various techniques the error is then fed back through the network. Using this information, the algorithm adjusts the weights of each connection in order to reduce the value of the error function by some small amount. After repeating this process for a sufficiently large number of training cycles the network will usually converge to some state where the error of the calculations is small. In this case one says that the network has learned a certain target function. To adjust weights properly one applies a general method for non-linear optimization task that is called gradient descent. For this, the derivative of the error function with respect to the network weights is calculated and the weights are then changed such that the error decreases (thus going downhill on the surface of the error function). For this reason back-propagation can only be applied to networks with differentiable activation functions.
4-4 Training the ANN (Optimization) Training is just the procedure of estimating the values of the weights and establishing the network structure, and an algorithm used to do this is called a “learning” algorithm. The learning algorithm is nothing more than some type of optimization algorithm, such as nonlinear programming, genetic programming,
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interval analysis. Once a network is trained, it can provide a response with a few simple calculations, which is one of the advantages of using an ANN.[105] Regardless of what algorithm is used to calculate the values of the weights, all of the training methods encompass the same general steps. First, the available data is divided into training and test sets. The following procedure is then used (called “supervised learning”) to determine the values of weights in the network: (1) For a selected ANN architecture, the values of the weights in the network are initialized often as small random numbers. (2) The inputs of the training set are fed to the network, and the resulting outputs are calculated. (3) Some measure of the error between the outputs of the network and the known (correct) values is calculated. (4) An optimization algorithm is executed in which the gradients of the objective function with respect to each of the individual weights may have to be calculated; the weights are changed according to the optimization search direction and step length to reach the initial stage of the next point. (5) The procedure returns to step 2. (6) Iteration terminates when the value of the error calculated using the data in the data set starts to increase, quite possibly from a local minimum. The purpose of partitioning the available data into the training and test sets is to evaluate how well the network generalizes (predicts) to domains that were not included in the training set. For nontrivial problems with multiple inputs and outputs, it is probably not possible to collect all of the possible input -output 65
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patterns needed to span the input -output space for a particular behavior or process to be modeled. Therefore, the network has to be trained with some subset of all of the possible input –output patterns. However, the training set must be representative of the domain of interest if the network has to learn (interpolating satisfactorily between measured points). If a suitable input set of data is not selected, the net may not predict well for similar data and may predict poorly for completely novel data. For extrapolation, as for example to predict the net output from input data taken outside the space of the training set of data, care should be taken, because blind extrapolation using any model is hazardous, particularly with an ANN. [105] In the learning process, there are several variables that have an effect on the ANN training. These variables are the number of iterations, learning rate, the momentum coefficient, number of hidden layers and the number of hidden neurons. To find the best set of these variables and parameters, all of those must be varied and the best combination chosen.[100]
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4-5 Back Propagation (BP) It is the most widely used in chemical engineering [108] which is a multilayer feed -forward network with hidden layers between the input and output. The simplest implementation of back propagation learning is the network weights and biases updates in the direction of the negative gradient that the performance function decreases most rapidly. [102] The back propagation (BP) algorithm was proposed in 1986 by Rumelhart, Hinton and Williams [109]. The weights can be adjusted by a gradient-descent-based algorithm. In order to implement the BP algorithm, a continuous, nonlinear, monotonically increasing, differentiable activation function is required. The two most-used activation functions are the logistic function equation (4.1) and the hyperbolic tangent function equation (4.3), and both are sigmoid functions [110]. n
4.2
net w0 ai wi i 1
Where: F net represents the actual output [113].
e net e net F (net ) net e e net
…(4-3)
During the learning process, weights in a network are adapted to optimize the network response to a presented input. The way in which these weights are adapted is specified by the learning rule. The most common rules are generalizations of the Mean Square Error (MSE) rule, equation (4.4), being the generalized delta rule or backpropagation, the most frequently used for supervised learning in feedforward networks. In supervised learning, a feed67
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forward neural network is trained with pairs of input-output patterns. For each input, the network produces an output. The accuracy of the response is measured in terms of an error defined as the difference between the current Yk and desired Tk output as in equation (4.4) [111].
T j , k n
MSE
m
j 1k 1
Y j,k
2 …(4-4)
nm
Where n represents the number of training patterns, m is the number of outputs,
T is the target value and Y is the actual value. Back-Propagation algorithm (BP) can then be used to adjust connection weights in the ANN iteratively in order to minimize the error calculated by equation (4.4)
[112]
. The error is propagated backwards from the output to the
input layer. Appropriate adjustments are made, by slightly changing the weights in the network. After weights have been adjusted, patterns are presented all over again. Error is calculated weights adjusted, and so on, until the current output is satisfactory, or the network cannot improve its performance any further [111]. The Back-Propagation (BP) algorithm to compute weights of neurons may tend to instability under certain operation conditions. To reduce the tendency to instability, Rumelhart in (1986) suggested adding a momentum term . is the momentum coefficient in the range of
( 0 <
